We describe in detail the histopathological features and MRI correlates in five MS patients following natalizumab-associated PML.
Our study was limited by the amount of tissue available for investigations and the small number of patients investigated. Additionally, the different time frames between the end of natalizumab treatment, diagnosis of PML, CSF tests, and IRIS may affect the pathology of the lesions, and must be taken into account when interpreting the data. Nevertheless, the homogeneous pathology in four out of five cases presented here gives a comprehensive picture of MS–PML–IRIS.
Histology reveals strikingly pronounced inflammation and no or only low numbers of virally infected cells in four out of five patients, indicating a strong immune response. The inflammation is dominated by CD8+ T cells; the abundance of plasma cells is remarkable. MRI shows enlarging and gadolinium-enhancing lesions, compatible with IRIS [2
]. Thus, we interpret the histopathology demonstrated here as PML-associated IRIS, as known from HIV-associated PML [15
]. However, inflammation in our study is even more pronounced than in HIV–PML–IRIS. This might be explained by the effectively reconstituted (and in MS patients otherwise intact) immune system after the withdrawal of natalizumab. MS–PML–IRIS was detectable in our analyzed patients even three months after PLEX/IA, and also occurred in patient 2, who had received neither of these therapies. IRIS typically occurs about 2–5 weeks after PLEX, necessitates repeated courses of intravenous steroids (suggesting that IRIS persists for several months), and has been described as occurring without PLEX/IA three months after the withdrawal of natalizumab [2
CD8+ cytotoxic lymphocytes (CTLs) are known to be the main effector cell in viral infection [6
]. In PML, an association between JC virus-specific CTLs and early control of the disease was shown [3
]. Moreover, CD4+ cells seem to play a critical role in viral defense. The occurrence of PML in HIV patients with low CD4 counts might be taken as an argument. JCV-specific CD4+ T-cell responses are found in PML survivors, but not in patients with active PML [5
A striking observation in our cases was the high number of plasma cells in MS–PML–IRIS. Interestingly, natalizumab treatment increases circulating pre-B and B cells in MS patients [10
]. The mobilization of JCV carrying pre-B cells from the bone marrow may play a role in natalizumab-associated PML pathogenesis [14
]. The role of plasma cells for viral defense in PML is debated. On the one hand, as PML develops despite the presence of JCV-specific immunoglobulin G (IgG), the humoral immune response seems incapable of preventing PML [9
]. An intrathecal synthesis of IgG-antibodies directed against the JCV viral protein 1 (VP1) is found in 76% of PML patients, but is not correlated with clinical outcome [26
]. On the other hand, a strong correlation between the plasma cell count in brain tissue and the humoral intrathecal immune response to the VP1 was found, suggesting a role for B cells/plasma cells and JCV antibodies in PML [26
]. Inflammatory PML with extensive mononuclear inflammatory infiltrates, including numerous plasma cells, was associated with a less severe neurologic illness [18
]. Finally, PML survivors also showed significant increases in JCV-specific IgG responses [7
In summary, a specific cellular as well as humoral immune response seems to be relevant for viral defense. In our study, CD3- and CD8-positive T cells, B cells, as well as plasma cells were negatively correlated with the number of virally infected cells, underlining their potential role in PML infection. We find higher plasma cell numbers in MS–PML–IRIS patients than in non-MS inflammatory PML cases, even when T-cell numbers are similar. Thus, we cannot rule out the idea that a least part of the plasma cell population is pre-existent and related to the natalizumab treatment.
The clinician who cares for an MS patient with PML who is deteriorating clinically and radiologically after stopping natalizumab therapy is faced with the differential diagnosis of PML–IRIS, ongoing PML, and MS exacerbation. Histologically, in MS exacerbation, actively demyelinating lesions that are otherwise typical of MS are expected [12
]. MRI is expected to show either homogeneously or ring-shaped enhancing lesions in typical periventricular or subcortical localizations. The histological characteristics of MS–PML–IRIS are clearly different from typical PML, as found in our fifth patient with natalizumab-associated PML. This patient was treated with immunosuppressive therapy before biopsy to combat IRIS. This case demonstrates the difficulty involved in finding the balance between suppressing clinical deterioration due to PML–IRIS, which may even lead to death, and halting the progression of PML [2
]. Our patient with active PML (patient 5) showed higher viral gene copies than the patients with PML–IRIS histopathology (see Table ). However, paraclinical parameters such as CSF JC virus load and peripheral CD4/CD8 cell counts do not help the clinician, as increasing viral titers as well as normal CD4/CD8 ratios can be found in both PML–IRIS and active PML [2
]. JC viral load is increased by greater than tenfold in MS patients with PML–IRIS and early MRI gadolinium enhancement [23
]. Although it seems unlikely, we cannot rule out that our patient with active PML showed IRIS histopathology in nonbiopsied lesions. MRI shows enlarging and gadolinium-enhancing lesions in both conditions. Contrast enhancement is a nonspecific sign of an impaired blood–brain barrier and can be seen with different patterns in inflammatory conditions of the brain. So far, it has not appeared possible to distinguish MS–PML–IRIS from ongoing PML in MS patients using only imaging data. However, PML lesions tend to show faint enhancement in the periphery of the lesion. More cases with confirmed pathology are needed to identify MRI patterns that differentiate these two entities.
In order to clarify the immunopathogenesis of MS–PML–IRIS, further investigations should focus on the role of plasma cells in JCV infection, as well as the effect of natalizumab on immune surveillance and inflammation within the CNS.
In summary, we report IRIS histopathology in four MS patients with natalizumab-associated PML. Histopathology is characterized by a pronounced inflammatory infiltrate with a predominance of CD8+ T cells and high numbers of plasma cells within lesions as well as adjacent grey and white matter. A biopsy was helpful in our cases to either establish the diagnosis of PML infection or to differentiate between ongoing PML, MS–PML–IRIS, and MS exacerbation. This guided the clinician to either stop medication (steroids, immunosuppressive therapy) in active PML or to further suppress inflammation with steroids in MS–PML–IRIS.